Field of the Invention
An acousto-optic dispersive light filter (hereinafter "AODLF") is a new acousto-optic spectroscopic device that exploits the optical birefringence properties of certain unique acousto-optical crystals, such as thallium arsenic selenide (Tl.sub.3 AsSe.sub.3) (hereinafter "TAS"). The structure and operation of an AODLF is disclosed in U.S. Pat. Nos. 4,639,092 and 4,653,869, both assigned to the assignee of the subject application, and hereby incorporated by reference.
An AODLF functions similar to a conventional diffraction grating. But, in an .AODLF the diffraction grating or spacing is electronically determined by the frequency of the acoustic signal applied to the AODLF. A crucial difference between a conventional diffraction grating and an AODLF is that the AODLF operates as a birefringent device, in which the polarization of the diffracted light is rotated 90.degree. with respect to that of the incident light, and the refractive indices are different in the acousto-optical crystal for the incident light and the diffracted light. The well-known equations which relate the angle of incidence (.theta..sub.i) and the angle of diffraction (.theta..sub.d) to the optical wavelength of the incident light and the acoustic frequency applied to the AODLF are: ##EQU1## wherein .theta..sub.i =the angle of incidence
.theta..sub.d =the angle of diffraction PA1 n.sub.i =refractive index for the incident light PA1 n.sub.d =the index for the diffracted light PA1 .lambda.=wavelength of incident light PA1 f=the acoustic frequency PA1 v=the acoustic velocity within the material
For TAS, n.sub.i is 3.339, n.sub.d is 3.152, .lambda. is preferably in the range of 1.3 to 17 microns and v is 10.sup.5 cm/sec. Operating an AODLF with incident light having wavelengths outside of this preferred range results in reduced efficiency since the AODLF material absorbs the light. The angles .theta..sub.i and .theta..sub.d are measured with respect to the acoustic wave front shown in FIG. 2.
FIG. 1 is a plot of equations of 1 and 2 as a function of the incident wavelength for an AODLF operating with an acoustic frequency of 231 MHz. The AODLF is operated at the minimum value for the incident angle .theta..sub.i as shown in FIG. 1. Operation of the AODLF at or near this minimum value of .theta..sub.i ensures that variations of the incident wavelength will have practically no affect on .theta..sub.i. The acoustic frequency and the wavelength of the incident light are related by the following equation. ##EQU2##
An important characteristic of any AODLF is the angle of incidence through which light may be applied to the AODLF, without degrading the resolution of the AODLF. This is known as the angular aperture of an AODLF. A large angular aperture or acceptance angle is desirable since this results in an increased light gathering power, and therefore increased sensitivity to weak light signals.
The maximum aperture of an AODLF is determined by the allowable phase mismatch, between the incident optical wave and the acoustic waves, beyond which the diffraction efficiency of the AODLF drops to one-half the value for exact phase matching (i.e., exact Bragg angle matching as given by equations 1 and 2). The allowable angular aperture of an AODLF is expressed as follows. ##EQU3## In equation 4, L is the interaction length between the incident light and the acoustic waves. Typically, L is the length of the acoustic transducer and the remaining terms of equation 4 have the same meaning as in equations 1 and 2.
As seen from equation 4, angular aperture is directly proportional to the refractive index n.sub.i and the acoustic velocity v. These quantities, however, are fixed for a given material. The angular aperture is also inversely proportional to the acoustic frequency f, and the transducer length L. The acoustic frequency f, is chosen so as to operate the AODLF in the region of minimum slope for .theta..sub.i as shown in FIG. 1.
From equation 4, the angular aperture can be made large by making the acoustic transducer length L small. However, it is desirable to have L large, because the diffraction efficiency of the AODLF and the drive power for the AODLF are related to L. Diffraction efficiency is a well known quantity and is discussed in I.C. CHANG, "Acousto-Optic Devices and Applications," IEEE Trans. on Sonics and Ultrasonics, Vol. SU-23 No. 1, pp. 2-21, Jan. 1976, and in Gottlieb et al., Electro Optic and Accoustic Optic Scanning and Deflection, Marcel, Dekker, 1985, at, for example, page 110, Equations 6.24 and 6.25. As the length L increases, the diffraction efficiency improves, for a given drive power density. Therefore, it is undesirable to make L small, because the power drive requirements therefor are great. In short, the smaller the length L, an greater the needed power density. As a result, with small transducer lengths the transducer tends to overheat. For example, if 5 watts are needed for an AODLF, applying this power to a large transducer provides a low power density. But, when applying it to a small transducer the power density may be too high for the transducer. Therefore, making L small limits the amount of power that can be applied to the transducer. As a result, the angular aperture of an AODLF cannot be greatly improved by varying the length of the transducer L.